1. Field of the Invention
The invention relates to a compact-size image-sensing device and, more particularly, to a CMOS (Complementary Metal Oxide Semiconductor) image-sensing device that can be minimized.
2. Description of the Related Art
In accordance with the popularity of image and sound multimedia and continuous introductions of digital image equipments, an image sensor begins to play a more important role in the industry. An image sensor is responsible for converting an optical image signal into an electrical signal and is classified into two categories of Charge Coupled Device (CCD) and Complementary Metal Oxide Semiconductor (CMOS) according to the different types of sensing device. Among them, the CCD image sensor is popularized for its stable quality and is most suitable for high-level digital cameras and digital camcorders. On the other hand, the technique of CMOS image sensor has moved into an application phase in recent years, and since it has the advantages of low power consumption, signal power, low cost and high integrity, it has been widely employed in low-level image products such as a low-level digital camera, computer camera, image telephone, videoconference equipment, mobile phone camera and Personal Digital Assistant (PDA).
A pixel cell is a base unit for converting light into electrical signal in an image sensor. FIG. 1 shows the configuration of a pixel cell in a conventional CMOS image sensor. As shown in FIG. 1, a pixel cell 1 includes a reset signal line 10, three transistors 12, 14 and 16, a photodiode 18, and a readout signal line 20 that is connected to the transistor 16, wherein the readout signal line 20 is also connected to a signal output bus 22, and the transistor 12 is connected to a reference voltage terminal 23. By this arrangement, every time before the exposure, the photodiode 18 is charged through the transistor 12 when the reset signal line 10 activated                the photodiode 18 can be reset back to its original reference voltage, while the reference voltage is provided by the reference voltage terminal 23. Next, at the time of exposure, the photodiode 18 will generate an voltage change when it is photosensitized. The signal from the voltage change is transmitted to the transistor 16 for storing by the transistor 14 that has a function of source-follower amplify. After the photosensitizing, the transistor 16 will transmitted the readout signal to output terminal 22 by means of the transistor 16 selected by the readout signal line 20 so that the image data readout can be further transmitted to an image processing circuit to proceed with subsequent image processing. Then, every time at the exposure, the actions of reset, exposure and readout will be repeated accordingly.        
However, a reset-signal control line and a readout-signal control line are provided in each pixel cell unit in the foregoing control circuit design of the conventional CMOS image-sensing device; therefore, not only will the circuit control become complicated, but the pixel cell size also will be larger in layout, which in turn will result in the chip size to be larger as well as the optical lens to be larger. Subsequently, the requirements of low cost and product minimization cannot be achieved, and the design cannot be competitive with the smaller pixel cell of the CCD, either.
In viewing the above problem, the invention provides a compact-sized image-sensing device, wherein the reset signal and the readout signal inside the image-sensing element are sharing the same signal line to cope with the above problem.